This invention relates generally to a polymer membrane type ion-selective electrode, and more particularly, to a polymer membrane type ion-selective electrode suitable for monitoring polyionic macromolecules such as heparin.
Polymer membrane type ion-selective electrodes are now routinely used in commercial biomedical instruments to measure accurately levels of clinical important small ions, such as Ca.sup.++, Na.sup.+, K.sup.+, Li.sup.+, H.sup.+, and Cl.sup.-, in undiluted whole blood. These ion-selective electrodes typically comprise a highly plasticized polymeric matrix material with an ion-exchange material or ion-complexing agent therein. The ion-exchange material may be a quaternary ammonium salt, such as tridodecyl methyl ammonium chloride (TDMAC).
Polyvinyl chloride (PVC) is a common polymeric membrane matrix material used in the art of solid-state or liquid-membrane electrodes for the detection of small ions (see, for example, U.S. Pat. No. 4,861,455 or Hartman, et al., "Chloride-Selective Liquid-Membrane Electrodes Based on Lipophilic Methyl-Tri-N-Alkyl-Ammonium Compounds and Their Applicability to Blood Serum Measurements," Mikrochimica Acta [Wein], 1978 II 235-246).
Efforts to develop similar sensors, including immuno-based biosensors, for the detection of large biomolecules, such as proteins or drugs, have thus far been unsuccessful. One of the most difficult problems has been identifying appropriate complexing agents and membrane chemistries that yield significant, specific and reversible electrochemical responses to the desired analyte. Even if a specific complexing agent is identified for a macromolecular biomolecule, whether the interaction with the macromolecular ion is strong enough to overcome the rather low mobility of a large ion to yield to significant electrochemical response remains in question. In theory, the sensitivity and selectivity of an ion-selective electrode membranes is governed by both the mobility of the analyte ion and the strength of the interaction between the ion-complexing agent and the analyte ion. In addition, strong interference resulting from a high concentration of small ions, such as chloride ions, in a blood sample may dictate the membrane's response.
An analyte of particular clinical significance is heparin, a polyanionic macromolecule. Heparin is the anticoagulant drug used universally in surgical procedures and extracorporeal therapies, and for the prevention of thromboembolism following surgery or childbirth. Heparin is a group of polydisperse (molecular weight ranges from 5,000 to 30,000 daltons) straight-chain anionic mucopolysaccharides called glycosaminoglycans having an average molecular weight of 15,000 daltons. Glycosaminoglycans are copolymers of sulfated (SO.sub.3.sup.-) and unsulfated uronic/iduronic acids alternating with glucosamine residues.
The major side effect of heparin administration is bleeding. In fact, a survey by the Boston Collaborative Drug Surveillance Program on drug-related deaths among in-patients indicted that heparin is the drug responsible for a majority of drug deaths in reasonably healthy patients. In view of this morbid potential, there is a great need for a means to continuously and accurately measure heparin levels in the bloodstream during surgical procedures. Unfortunately, there currently is no method suitable for direct and rapid determination of the physiological heparin levels. Presently available heparin assays, such as the Activated Clotting Time, are all based on blood clotting time. Further, the prior art assays are not specific to heparin and lack speed, accuracy, consistency, and a defined biochemistry. Further, since the clotting time based heparin assays can not directly measure the blood heparin level, the role of heparin in the associated bleeding complications and the mechanism of the "heparin rebound" phenomenon have never been identified. There is, therefore, a need for a means of directly measuring the levels of heparin in the blood in both clinical practice and medical research.
The quaternary ammonium salt TDMAC is known to bind or complex with heparin. In fact, it is well-known in the medical arts to fabricate thromboresistant biomaterials by heparinizing the surface of a TDMAC-coated or impregnated polymer. TDMAC shares significant structural similarity to polybrene, a synthetic polyquarternary ammonium salt, considered to be one of the most potent heparin antagonists. Although TDMAC has been used as the anion-exchange material in conventional membrane electrodes for the detection of small ions, the art is totally devoid of any suggestion that macromolecules, such as heparin, could be directly detected with TDMAC-doped PVC membranes.
It is, therefore, an object of this invention to provide an electrochemical sensor for ionic macromolecules.
It is another object of this invention to provide an electrochemical sensor for direct measurement of heparin in whole blood or plasma.
It is also an object of this invention to provide an electrochemical sensor for direct measurement of heparin in whole blood or plasma which is accurate over the expected clinically relevant concentration range.
It is a further object of this invention to provide an electrochemical sensor for direct measurement of heparin in whole blood or plasma which possesses adequate dynamic response characteristics, i.e., responds rapidly to a change in ion concentration and returns promptly to baseline, so that it is suitable for continuous in vivo monitoring.
It is additionally an object of this invention to provide a polymeric membrane electrode having specific selectivity for heparin macromolecules even in the presence of Cl.sup.- and other anionic impurities.